1. Field of the Invention
The present invention relates to an organic electrolyte battery, and more particularly to an organic electrolyte battery of a cylindrical shape with a PTC device for the protection of the battery.
2. Description of the Prior Art
In conventional cylindrical organic electrolyte batteries, as the negative electrode, lithium or a lithium alloy is used and, as the positive electrode, there is used a forming body of a positive electrode agent using an active material of metal oxide, such as manganese dioxide or metal hydrosulfide such as titanium dioxide. The negative electrode and positive electrode mentioned above are respectively shaped in the form of a sheet and laminated together through a separator and the laminated sheets are wound in a roll, which is contained in a cylindrical battery case. As the electrolyte there is used an organic electrolyte made of an organic solvent, such as propylene carbonate, tetrahydrofurane, dimethoxyethane or dioxilane containing solved lithium salt, such as lithium perchlorate.
As mentioned above, in the cylindrical organic electrolyte battery, the positive electrode and negative electrode are formed in a roll by winding both electrode sheets and the roll is accommodated in the container. The area of both electrodes is very large and there is a possibility of flowing a heavy current in the battery. If such a heavy current is continued, the battery temperature may be undesirably raised and the separator, made of a microcellular polypropylene film, may be harmed, thereby resulting in an inner shortcircuit of the battery. In the worst case, the battery may catch fire or may blow up.
In order to avoid such damage to the battery as mentioned above recently, it has become popular to attach a PTC device to each battery. In this arrangement, when a heavy current flows in the battery, the plastic resin layer of the PTC device becomes hot and the resistance of the hot resin layer of the PTC device becomes high, thereby suppressing the heavy current flow.
The conventional PTC device 2 mentioned above, as shown in FIG. 1, essentially consists of a resin layer 2a having a PTC characteristic and two metal layers 2b1 and 2b2 provided on both surfaces of the resin layer 2a, as shown in FIG. 2 with respect to the embodiment of the present invention. Such PTC devices are commercially sold from RAYCHEM CORPORATION with the trade name "polyswitch". PTC is an abbreviation of "Positive Temperature Coefficient" and the expression "PTC property" as used herein means such property that the electric resistance of the device increases extremely when the PTC device is heated.
In the conventional cylindrical organic electrolyte battery with the PTC device, which are commercially available, the PTC device 2 is installed in a sealing member 18, as shown in FIG. 1.
It seems that such a structure, as mentioned above, is adopted because, when the PTC device is disposed in the sealing member 18, it is not necessary to heat the PTC device and the resin layer 2a of the PTC device 2 is not thermally deteriorated and therefore the PTC characteristic of the PTC device is not harmed.
There are further problems in the conventional battery with the PTC device as mentioned below. In the cylindrical organic electrolyte battery, gas is generated inside of the battery due to an internal shortcircuit of the battery, whereby the internal gas pressure of the battery abnormally rises and the battery may explode. In order to prevent such battery explosion, there is provided a flexible, thin sheet 21 in the sealing member 18 and, in addition, a cutting blade 22a and a gas ventilation port 22b are formed on a terminal plate 22. Further, a gas ventilation opening 19a is defined in the sealing plate 19. Therefore, the PTC device 2 must be made in a round shape. Furthermore, since the PTC device 2 must be disposed in the sealing member 18, the sealing property of the sealing member 18 may deteriorate and the PTC device 2 can not be operated as desired due to an inner shortcircuit. When the gas is generated in the battery, the gas reaches the flexible thin sheet 21 through the gas ventilation opening 19a of the sealing plate 19 pushing up the central portion of the flexible thin sheet 21. The pushed up portion of the flexible thin sheet 21 comes into contact with the cutting blade 22a and is broken., thereby allowing the gas generated in the battery to pass toward the outside of the battery through the ventilation port 22b, so that the battery can be prevented from exploding. However, in order to assure the normal operation of the explosion preventing device, such as the breakage of the flexible thin sheet 21, it is necessary to arrange the PTC device 2 in such a manner that the PTC device 2 does not prevent the upward deformation of the flexible thin sheet 21 when the gas is generated in the battery. Therefore, the PTC device 2 must be shaped in a circular shape. As a result, the effective area of the PTC device becomes small and the resistance thereof must be high, therefore, the internal resistance of the battery increases.
In addition, in order to cause the current to flow through the PTC device 2 before the current reaches the terminal plate 22, it is necessary to avoid a direct contact between the sealing plate 19 and the terminal plate 22. For this purpose, in the conventional method, there is disposed an insulation packing 26 in the sealing member 18 so as to cover the PTC device 2 and the outer periphery of an annular metal plate 27, having the flexible thin sheet 21 contacted with the sealing plate 19, which is arranged so as not to directly or electrically contact the terminal plate 22 in addition, the flexible thin sheet 21 contacts the metal plate 27, so that the current can flow through a path consisting of the sealing plate 19, flexible thin sheet 21, metal plate 27, PTC device 2, and terminal plate 22, in the order as described. The sealing member 18 is assembled, fastening the insulation packing 26 by bending the peripheral edge portion of the sealing plate 19 toward the inside of the battery. In this structure, since the PTC device 2 is shaped as a plate, the flanged edge portion of the terminal plate 22 must be a flat plate shape. In addition, since the insulation packing 26 is not disposed between the peripheral edge portion of the sealing plate 19 and the peripheral edge portion of the flexible thin sheet 21, sealing of the sealing member 18 tends to be lowered, whereby the electrolyte of the battery is apt to leak outside of the battery passing the interface between the insulation packing 26 and the metal plate 27 or PTC device 2, further passing the gas ventilation port 22b.
Moreover, the resin layer 2a of the peripheral edge portion of the PTC device 2 tends to be crushed by the fastening force applied at the time of assembling the sealing member 18, whereby the metal layers 2b1 and 2b2 disposed on both surfaces of the resin layer 2a, contact, resulting in an inner shortcircuit and the PTC function may be lost.